This invention relates to liquid crystal display devices and more particularly to such devices that utilize a thin body of twisted nematic liquid crystal material supported between two substrates.
Nematic liquid crystal materials, when in a mesomorphic phase, have the useful property of effecting the direction of polarization of incident linearly polarized light. This property can be modified or altered by the influence of an electrical potential or a magnetic field. As well known in the art, various organic nematic compounds may be formed into thin layers and subjected to an electrical field, the selective application of the field serving to change the light reflective or light transmissive properties of the nematic compounds, thereby making possible various optical display devices. The use of twisted nematic liquid crystal materials have been found to be particularly desirable for optical cells of the so-called field effect type because they require lower voltage, and less power. However, the operation of twisted nematic liquid crystal cells of the field effect type requires the effective homogeneous alignment of the liquid crystal molecules. That is, the relatively long molecules must be constrained to lie in a cooperative orientation, parallel to the inner surfaces of the spaced apart plates of the cell. If two transparent plates are provided each with inner surface characteristics such that the liquid crystal molecules are caused to be oriented in parallel alignment on its own plate but at right angles with respect to the other plate, then the resulting effect will be an optical media which rotates the plane of linear polarization by 90.degree.. Similarly, if the two elongated molecules directions are aligned 45.degree. with respect to each other, the resulting nematic liquid will rotate the plane of linearly polarized light at 45.degree.. Similarly, any amount of optical rotation can be obtained. The liquid crystal material is elastically deformed by intermolecular interaction, such that the long axes of the nematic material are oriented in a helical manner relative to each other between the plates. When an electric field is applied to the device, the structure will untwist at a well defined voltage and allow light transmissions when the device is between parallel polarizers.
In order to achieve the necessary parallel orientation or alignment of molecules on the cell plates, it was necessary in prior devices to modify the transparent plate surfaces by rubbing their inner surfaces unidirectionally to provide an alignment axis and an optic axis in the same direction. Mere clean glass or clean transparent electrode surfaces alone did not secure the desired alignment. Later, another effective means for achieving this molecule alignment of nematic-phase liquid crystal material involved the vacuum deposition of a thin film of silicon monoxide (SiO) at a high angle of incidence on the inner surface of each transparent cell plate. This procedure, now well known in the art, was developed by J. L. Janning and is described in Applied Physics Letter 21, 173 (1972). However, the Janning technique for obtaining alignment was not always successful in all cases, nor did it provide for an alignment that was always uniformly high at the boundaries between a glass surface and the transparent conductive electrode areas on the cell plates. It is believed that the SiO layer did not produce reliable alignment because it did not grow at the same rate and with the same surface texture on the conductive pattern region of the cell plates as it did in the clear glass regions of the plate. In addition, the molecule alignment occurring at the interfaces of conductive and nonconductive regions was either random or in some cases essentially homeotropic using the SiO layer. Such areas would appear as smudges on the display devices the same as regions of normally activated areas. Also, such imperfections often resulted in the appearance of strong first-order optical interference colors or patterns which made it difficult or impossible to view the display.
Another problem which arose relative to the direct application of the SiO layer to the patterned conductive areas on the display plates was that the SiO layer was not impervious to ionic currents which could therefore penetrate it and cause electro-chemical damage to the conductive oxide layer beneath. In extreme cases the SiO layer itself could be damaged by such ionic penetration which would also result in degradation or complete malfunction of the cell.